Gearbox cracks

I would replace the shell, both Norvil and RGM offer strengthened gearbox shells, a bent mainshaft tells me this gearbox has not had a happy life.

The 850 commando's were torquers and were probably too much for the AMC box.

Toodles,

Albert

I agree with Albert - I rebuilt my 850 gearbox a few years back using the RGM 'strengthened-shell' - works fine. I seem to recall that at the time this shell was actually cheaper than the standard item.

Cheers,

Mark Woodward

The "standard item" is actually a strengthened version of the original gearbox shell, made along the lines of the Mk3 gearbox shell these days. I suppose this is what you bought: http://www.nortonmotors.de/ANIL/Norton%20Website/norton/parts-list.php?Model=n850_1&Plate=007&Part=1

David, a tiny crack in the web between the bearings wont hurt.Mine was cracked there for at least fifteen years,the crack didnt seem to spread.A tiny crack in the shaft will cause rapid failure, so either have it crack tested or replace it.The crack in the shell will start to leak oil if it gets worse,a broken mainshaft may wreck your primary case ,or other gears in the box. You will also be walking home. Regards John.

If you don't wind up in a ditch wondering what hit you, I'd replace both items because I'm paranoid that way.

Albert

From an American forum: How to change gear: Open wide to valve bounce,stomp the gear pedal,hit the kill button ,no clutch. Says he s had more n a dozen Commando s, sells em on to export buyers. Now you know why your shafts bent. Regards John.

Previously wrote:

Sorry hit the wrong button and sent it before writing my message. I took my 850 MK2A gearbox apart at the weekend as the clutch was doing a strange movement. I suspected sleeve gear bush wear but on getting the shafts on centres also found the mainshaft to be bent. A little work on the fly press has got it running fairly true so it may reusable.

However, removing the two bearings has revealed a small crack between the two housings. It casing design looks as if this is inevitable, but can I continue to use the housing or have I really got to consider a new gearbox shell ? There is no sign of the bearings rotating in their housings, although it is evident that the Layshaft bearing had broken at one time, but since I've had the bike it has always had the roller bearing upgrade.

What is the acceptable run out on a mainshaft?

Anything less than.005 TIR is ok, after straightening your shaft should be checked for cracks if bend exceeded .015 TIR before straightening.Regards John.

Hi John,

Excuse my ignorance and just out ofcuriosityas I am not likely to try tostraightenany gearbox shafts. But what is TIR ?

Regards

Tony

TIR stands for Total Indicated Runout, that is, the total runout shown on a dial indicator when the shaft is being checked.